Every Post in WordPress is filed under one or more Categories. Categories allow the classification of your Posts into groups and subgroups, thereby aiding viewers in the navigation and use of your site.

You can try using BulkPress. It allows you to create hundreds of categories very easily. Follow these steps:
Install and activate the plugin.
In the left side bar, hover your cursor on Bulkpress … . Then click
on Terms.
A new page will appear. In Taxonomy field, choose Category.
Insert your Categories in the Terms field.
Choose your desired Parent, if required.
Finally, click on the Add Terms button.
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You can try using Term Management plugin. You can use it to bulk merge your duplicate categories into one. Follow these steps:
Install and activate the plugin.
Go to Posts > Categories.
Select the … categories that you want to merge. Then select the merge
option from the bulk option.
Type the name to which you want to merge these categories. Finally,
click on the Apply button.
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